Fabrication of mass-produced electronic items such as printed circuit boards (PCBs) typically involves utilizing both sides of the circuit board as well as using one or more internal layers within a laminated stack of integrated circuit boards. The opposite sides, internal layers, and adjacent stacked boards are electrically connected by means of vias. Vias are holes that are drilled or otherwise formed to provide an electrical path that connects between layers of printed circuit boards. In order to pack a board more densely with components, a “via in pad” strategy is even used, forming the via within a connecting pad in the tracing pattern.
In order for a via to effectively provide a suitable interconnection, the via is typically electroplated or otherwise metallized along its side walls and outer edges. It can be disadvantageous to leave the via hole without some type of filling, however. A simple unfilled via leaves a perforation or hole in the circuit board upon which it is difficult to place an electrical component. Thus, a conductive via fill is useful for component mounting. Copper plating or conductive via fill is also used in order to improve heat dissipation.
There are a number of options for filling a via. Electroplating is one approach that is commonly used for this purpose, but can be costly in terms of time and complexity. The electroplating process requires harsh chemical baths with consequent handling of toxic materials. Electroplating for vias uses a specially designed plating bath and can require as long as 5 hours for a typical board.
As an alternative, PCB fabricators can use a conductive paste for via filling. Commercially available pastes, such as conductive via plug paste CB100 from DuPont Microcircuit Materials, Research Triangle Park, N.C. and conductive pastes from Tatsuta Electronic Materials, Osaka, Japan are epoxy based and can contain some micron-sized copper and silver particulate. Epoxy based pastes, however, have a number of shortcomings. The cost of epoxy-based via fill materials is relatively high, due in part to the silver content. Even with the metal particulate additives, conductivity is significantly below that of bulk copper. Pastes of this type generally have a relatively short working time and must be stored in sub-zero temperatures. Epoxy fillers can also outgas, expand, or contract affecting the integrity of the plated thru hole.
Still other types of non-conductive fillers have been used for via fill, including nonconductive pastes that pack the cavity but do not provide electrical paths or provide any thermal benefit.
Recent advances in printed electronics provide solutions that reduce the cost, complexity, and energy requirements of conventional conductive circuit trace deposition methods and expand the range of substrate materials that can be used for making circuit boards. For printed electronics, materials can be deposited and cured at temperatures compatible with FR4 and plastic substrates and can be handled in air. In particular, advances with nanoparticle-based inks, such as silver, copper, and other metal nanoparticle-based inks, for example, make it feasible to print electronic circuit traces and structures using standard additive printing systems such as inkjet and screen printing systems. Advantageously, nanoparticle-based inks have lower curing temperatures than those typically needed for bulk curing where larger particles of the same material are used.
Commercially available systems for curing nanoparticles typically employ heat from convection ovens or Xenon flash illumination energy. In such illumination systems, the Xenon lamps emit pulsed light that is directed onto films of nanoparticles to be cured. High light energy levels are required for nanoparticle curing. Exemplary nanoparticle-based inks, such as Intrinsiq Material Inc. product CI-003, a copper nanoparticle based inkjet ink, or product CP-003, a copper nanoparticle-based screen print ink, can be sintered through the use of photonic energy from Xenon lamp or other illumination, provided that the illumination system delivers adequate energy to volatilize coatings and additives used in the ink formulations and to sinter and cure the inks over large surface areas. Alternatively, nanoparticles may also be cured by means of laser illumination. The laser provides concentrated energy with a restricted spatial profile, allowing greater penetration of curing energy into the printed paste, thereby allowing higher-resolution features to be formed.
Even with the advances of nanoparticle-based inks and laser energy curing techniques for forming circuit traces, however, via formation remains difficult, time-consuming, and costly. The problem of via fill has not been adequately addressed. It can be appreciated that there is a need for via formation methods that allow improved conductivity and are compatible with methods for printing electronic traces on PCBs.